Neverwinter Nights 1 is a great coop game. Dungeon siege is too. You can always play with a friend against bots in Counter-strike or Battlefield and most other FPS and RTS games. Guild wars is always fun with a group of friends.

Umm, why has no one said SWAT 4 yet? Using beanbag rifles and pepper spray to take down terrorists is awesome. And the COOP is fun. There was also a huge fanbase behind it at the time and many maps have been release for it. You'll never tire from only playing the same ol maps

*edit*

dont forget to get the expansion pak if you get SWAT, it adds a lot of cool features for online/coop such as the AK47, several new game modes, and the ability to choose light, med, or heavy armour!!!